The present invention relates to generation of ultraviolet (UV) laser radiation. It relates in particular to generation of ultraviolet laser radiation by generation of the third and fourth harmonics of fundamental laser radiation having a wavelength in the near-infrared (NIR).
UV laser radiation is used in optical lithography for patterning photoresist for lithographic masking operations. This patterning may be accomplished by exposing photoresist through a mask having transparent regions therein or, in a xe2x80x9cdirect writingxe2x80x9d mode by steering a focussed beam of UV radiation over the photoresist to define the pattern. Generally, the shorter the wavelength of the radiation, the greater is the resolution and accordingly the smaller the feature size. In this direct writing mode, the quality of the beam can be as important as the wavelength of the beam for obtaining highest possible resolution. Accordingly, the beam is preferably provided by a single operating mode of the laser. The radiation of the beam is preferably either continuous wave (CW radiation) or rapidly pulsed radiation delivered at a sufficiently high pulse rate relative to the pulse duration that it has essentially the same effect as CW radiation.
One preferred prior-art approach to generating a high-quality beam of quasi-CW UV radiation is to provide a diode-laser-pumped, passively modelocked laser resonator delivering NIR radiation at a wavelength of about 1064 nanometers (nm) from a solid state-gain medium such as Nd:YAG or Nd:YVO4. The NIR radiation is then converted to ultraviolet radiation by first doubling and then either tripling or quadrupling the frequency of the NIR radiation. This is done using optically-nonlinear crystals located outside of the laser resonator. Passive modelocking of the NIR laser resonator is typically accomplished using a saturable absorber or the like. The passively-modelocked radiation is typically delivered at a frequency of about 250 megahertz (MHz) or greater.
A problem with this prior-art approach is that it is relatively inefficient. By way of example, in order to generate 5.0 Watts (W) average power of 355 nanometer (mn) wavelength radiation by frequency tripling quasi-CW 1064 nm radiation, the 1064 nm radiation must have an average power of about 25 W to 30 W. This would require a pump-light (diode-laser array) power of about 50 W to 60 W. Conversion to 266 nm radiation by frequency quadrupling would be even less efficient. There is clearly a need for a more efficient-source of quasi-CW UV radiation.
The present invention is directed to providing a modelocked harmonic-generating laser. In a general aspect, the inventive laser comprises a laser resonator terminated by first and second mirrors. A gain-medium is located in the laser-resonator. An optical pump-light source provides pump-light for energizing the gain-medium, thereby causing fundamental radiation having a fundamental frequency to circulate in the laser resonator. A first optically-nonlinear element is located in the laser resonator. The first optically-nonlinear element is cooperatively arranged with the first mirror to provide passive modelocking of the fundamental-frequency radiation by sequentially converting a first portion of the circulating fundamental radiation to second-harmonic radiation and reconverting a first portion of the second-harmonic radiation to fundamental radiation. A second optically-nonlinear element is arranged to convert a second portion of the second-harmonic radiation to either third-harmonic radiation or fourth-harmonic radiation.
In a first particular aspect of the inventive laser, the second optically-nonlinear element is arranged to convert the second portion of the second-harmonic radiation to third-harmonic radiation by mixing it with a second portion of the fundamental radiation. In one preferred embodiment in accordance with this first particular aspect of the inventive laser, the second optically-nonlinear element is located outside of the laser resonator and the second portions of the fundamental and second-harmonic radiation are transmitted out of the laser resonator by the first mirror. Preferably the transmissivity of the first mirror for the fundamental and second-harmonic radiations is arranged such that the power of the transmitted second portions of the fundamental and second-harmonic radiation is about equal. In another preferred embodiment, the second optically-nonlinear element is located inside the laser resonator between the first mirror and the first optically-nonlinear element.
In a second particular aspect of the inventive laser the second optically nonlinear element is arranged to convert the second portion of the second-harmonic radiation to fourth-harmonic radiation. In one preferred embodiment in accordance with this second particular aspect of the inventive laser, the second optically-nonlinear element is located outside of the laser resonator and the second portion of the second-harmonic radiation is transmitted out of the laser resonator by the first mirror. In another preferred embodiment in accordance with this second particular aspect of the inventive laser, the second optically-nonlinear element is located inside the laser resonator between the first mirror and the first optically-nonlinear element.